BIOL 151 Lecture Notes: Cell Signaling, Stem Cells, and Signal Transduction Dynamics

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Big-picture Questions about Cell Signaling

  • How do cells send and receive signals?
  • How is signaling dynamic and fluctuating between active and inactive states?
  • How does signaling affect gene expression?
  • What different types of signaling pathways exist?
  • How does cell signaling control cell division?
  • Core term: Signal Transduction

Reading assignments (context for study)

  • Chapter 9 - Cell Signaling
  • Section 20.1 Genetic Programs of Development - stem cell intro
  • Text: How Life Works, 4th Ed.

Major components of all cells (basics you should memorize)

  • Cell membrane (plasma membrane): barrier between inside and outside of the cell
  • Nucleus with genetic material
  • Receptors on the cell surface that receive information from other cells
  • Various organelles for gene expression, production of proteins, carbohydrates, lipids, and energy metabolism

Tissues, organs, and cellular interaction (concepts)

  • Tissues are made of cells that interact to form functional organs
  • Reference: overview linked to Medical News Today article on tissue organization

Developmental lineage: a single fertilized egg gives rise to many cell types

  • Examples of cell lineages include bone, liver, kidney, sperm, lung, brain, heart, blood, muscle, pancreas, egg

Cellular turnover and cell lifespans (human body scale)

  • Total human cells: ~37,000,000,000,000 (37 trillion) cells
  • Dividing cells at any moment: ~330,000,000,000 (330 billion) cells
  • Replacement rate: ~1% of total cells divide per day
  • Composition of dividing cells:
    • ~86% are blood cells (roughly 90% of all cells are blood cells)
    • ~12% are gut cells (0.5% of all cells are gut cells)
    • ~2% are everything else
  • Lifespans of select cell types and tissues (examples):
    • Brain cells: > years to decades (often 200+ years, depending on type/region)
    • Eye lens cells: lifetime
    • Egg cells: ~50 years
    • Heart muscle cells: ~40 years
    • Intestinal lining cells: ~3–5 days
    • Skeletal muscle cells: ~15.1 years
    • Fat cells: ~8 years
    • Red blood cells (RBCs): ~120 days
    • White blood cells (various types): ~10 hours to 10 days
    • Liver cells: ~10–16 months
    • Pancreas cells: ~1 year
    • Hematopoietic stem cells: ~5 years
  • Source illustrates rapid turnover in some tissues vs. long-lived neurons and lens cells; important for understanding signaling needs to regulate replacement and maintenance

Anatomy and niche concepts relevant to signaling

  • Bone marrow: Hematopoietic stem cells reside in a porous matrix; high rate of cell division; produces red and white blood cells and immune cells
  • Intestine: Intestinal villi increase surface area for nutrient absorption
  • Intestinal stem cell (crypt) niche: Signaling systems regulate stem cell division and differentiation of progenitors into absorptive enterocytes, enteroendocrine, Goblet cells; signaling components include Noggin, BMP/Bmpr1a, Wnt, PI3K/Akt, Pten, Smads, EphB2, and mesenchymal signals; crypt Paneth cells and pericryptal fibroblasts produce signals; proliferation at the crypt base with proliferating progenitors expanding across the crypt-villus axis
  • Hair follicle: Signaling in bulge stem cell niche involves Noggin, BMP4, Wnt, Lef/Tcf, c-Myc, Rac1; differentiation and proliferation drive hair cycle and follicle renewal
  • Liver: lineage-traced liver stem cells give rise to progeny expanding across liver lobule; stem cell dynamics studied via lineage tracing in mice

Stem cells and self-renewal vs differentiation (key concepts)

  • Stem cells receive signals that regulate self-renewal divisions; some signals promote self-renewal, others promote differentiation
  • Self-renewal divisions can be asymmetric: one daughter remains a stem cell, the other becomes a transit amplifying cell
  • Transit amplifying cells divide rapidly to expand a partially mature population; later signals drive differentiation into mature cell types
  • Once mature, most cells divide rarely or not at all
  • Adult stem cells are pluripotent only within a limited lineage context (e.g., mesenchymal stem cells can become muscle, skin, fat, bone, connective tissue, but not all tissue types)

Core signaling and stem cell biology: essential questions

  • How do cells “know” what to do?
  • What kinds of information do cells receive?
  • How is information processed?
  • What changes occur in the cell in response to signaling?
  • How does signaling affect gene expression patterns?
  • How does signaling control cell division?
  • How does signaling regulate differentiation into mature cell types?
  • How are long-term cell-type patterns stabilized through gene expression changes?

Signal transduction basics: pathway structure and flow

  • General model: signal-producing cells release signaling molecules → diffuse to target cells with receptors → receptor activation triggers a signal transduction cascade → cellular response (e.g., enzyme activation, gene expression) → signal termination to reset system for new signals
  • A typical ligand-receptor interaction initiates a cascade of intracellular events that culminates in transcriptional changes or acute responses

Signaling network types and overview diagrams (visual cues from slides)

  • Common depiction: ligand binds receptor → receptor activation → intracellular signaling cascade (kinases, second messengers) → transcription factors regulate gene expression in the nucleus → cellular response
  • Some slides show signaling involving MAP kinase cascades and interactions with numerous kinases, phosphatases, and transcription factors
  • Complex networks include cross-talk between pathways (e.g., Wnt, Notch, TGF-β families) and integration with metabolic and growth signals

Kinase cascades and key biochemical principles

  • Core idea: kinases transfer phosphates from ATP to target proteins; phosphatases remove phosphates
  • Basic reactions:
    • Kinase action: ext{Protein} + ext{ATP}
      ightarrow ext{Protein-P} + ext{ADP}
    • Phosphatase action: ext{Protein-P} + ext{H}_2 ext{O}
      ightarrow ext{Protein} + ext{Pi}
  • ATP as the phosphate donor is abundant; phosphates attached are covalent and robust
  • Key observation (from notes): kinases and phosphatases compete; the net phosphorylation state depends on their relative activities
  • In many BIOL 151 problems, the phosphatase can “win” if it acts faster or more effectively than the kinase, leading to an unphosphorylated (off) state

Receptor activation and signal transduction in a simplified Kinase Cascade

  • Receptors: two subunits may be inactive without ligand; ligand binding brings subunits together to activate kinase activity
  • Sequential activation: TK1 phosphorylates TK2; TK2 phosphorylates TFa (a transcription factor)
  • Nuclear action: Activated TFa moves to nucleus and modulates gene expression
  • Common consequences: transcription of target genes, producing new proteins that alter cellular behavior

Example: Insulin receptor phosphorylation as a model of activation

  • Inactive receptor is phosphorylated upon activation; the phosphorylation state propagates through the cascade to drive metabolic and growth responses

Additional signaling details and practical considerations

  • Phosphorylation is dynamic and reversible; turnover and degradation of phosphoproteins shape signaling duration
  • Phosphorylation state is tied to protein stability and turnover; ubiquitin-dependent proteolysis provides a mechanism to remove damaged or misregulated signaling proteins
  • The ubiquitin-proteasome system selectively marks proteins for degradation via ubiquitin tags (often through Lysine-48 or Lys-11 linkages) and shuttles them to the proteasome for destruction
  • Proteasome architecture and function: regulatory particles (RP) recognize ubiquitinated substrates, feed them into the core particle (CP) where proteolysis occurs
  • The E1–E2–E3 ubiquitin cascade: E1 activates ubiquitin, transfers to E2, and E3 ligases transfer to the substrate; deubiquitinases (DUBs) remove ubiquitin; there are hundreds to thousands of E3s, enabling substrate specificity
  • Protein turnover is coupled to gene expression: when signaling networks are active, new proteins are synthesized; when signaling ceases, degradation continues and cells may return to baseline states unless gene expression has established a new steady state
  • Gene expression states (ON/OFF) determine whether a protein is produced; signaling can switch ON genes for transcription factors, receptors, kinases, and phosphatases; conversely, gene OFF leads to degradation of existing proteins without replenishment

Pathway dynamics: functional vs nonfunctional vs temporary signaling

  • Functional pathway: signal binding to receptor leads to changes in gene expression that persist while the signal is present or as long as downstream gene products exist
  • Nonfunctional pathway: signal may bind receptor but transduction is blocked; no gene expression change
  • Temporary pathway: initial gene expression changes occur, but stop even though signal remains bound; downstream signaling components may be turned off or degraded

Practical scenarios and reasoning checks (from slides)

  • Question: If TK3 is added and constitutively active, what happens to pathway behavior?
    • Likely outcome: a positive feedback-like effect could lock the pathway on; TK3 targets upstream elements, creating sustained signaling
  • A new kinase introducing a positive feedback loop could maintain pathway activation even after signal removal if it phosphorylates upstream components
  • If a phosphatase targets a downstream node, it can counteract upstream kinase activity; the balance of activities determines net phosphorylation state and pathway output

Signaling and gene expression coupling: a two-layer view

  • Layer 1: Signal transduction cascades (post-translational control)
    • Activation/inactivation of kinases and phosphatases
    • Phosphorylation states influence transcription factor activity, protein localization, and enzyme activity
  • Layer 2: Gene expression changes (transcriptional control)
    • Activated transcription factors move to the nucleus and drive expression of target genes
    • Gene products (receptors, kinases, phosphatases, metabolic enzymes) feed back into signaling networks to shape future responses
  • The interplay between these layers governs how cells respond, adapt, and stabilize new cell states

Signal termination and resetting the system

  • Termination can occur via receptor desensitization, receptor endocytosis, and degradation of signaling components
  • Phosphatases and phosphatase-targeted degradations help reset phosphorylation states
  • Ubiquitin-dependent proteolysis removes signaling proteins, clearing previous signals and enabling new signaling cycles

Detailed look at protein turnover and ubiquitin-proteasome system

  • Core idea: selective degradation adjusts protein levels to tune signaling output and regulate cell state
  • Ubiquitin tagging steps (simplified):
    • E1 activates ubiquitin in an ATP-dependent manner and transfers it to E2
    • E3 ligases transfer ubiquitin from E2 to the substrate; chain formation (often Lys48 or Lys11 linked) marks for proteasomal degradation
    • Deubiquitinases (DUBs) remove ubiquitin from substrates, rescuing them or altering fate
  • Proteasome structure (high level):
    • 19S regulatory particles recognize ubiquitinated substrates
    • 20S core particle (CP) carries the proteolytic sites
    • Substrates are unfolded and threaded into CP for degradation
  • Consequences for signaling: selective degradation of signaling proteins (e.g., receptors, kinases, transcription factors) provides precise temporal control and prevents runaway signaling

Practical implications and real-world relevance

  • Turnover and signaling are tightly linked to development, tissue homeostasis, and regeneration
  • Dysregulation can contribute to aging, cancer, and neurodegenerative diseases due to uncontrolled signaling, failed turnover, or misfolded proteins
  • Understanding the balance between kinases, phosphatases, and ubiquitin-mediated degradation informs drug targeting (kinase inhibitors, phosphatase modulators, proteasome inhibitors)

Quick reference: key quantities and concepts to memorize

  • Cell counts and division metrics:
    • Total human cells: 3.7×10133.7 \times 10^{13}
    • Dividing cells: about 3.3×10113.3 \times 10^{11} (330 billion) at any moment
    • Daily turnover: approximately 1%1\% of all cells divide per day
  • Lifespans (selected examples): RBCs: 120 days120\text{ days}; White blood cells: from hours to days; intestinal lining cells: 3–5 days3\text{–}5\text{ days}; eggs: ∼50 years\sim50\text{ years}; brain cells: long-term (years to decades)
  • Signaling cascade steps (typical): receptor activation → TK1 → TK2 → TFa (nuclear transcription factor) → transcriptional response
  • Core biochemical reactions:
    • Kinase-catalyzed phosphorylation: Protein+ATP→Protein-P+ADP\text{Protein} + \text{ATP} \rightarrow \text{Protein-P} + \text{ADP}
    • Phosphatase-catalyzed dephosphorylation: Protein-P+H2O→Protein+Pi\text{Protein-P} + \text{H}_2\text{O} \rightarrow \text{Protein} + \text{Pi}
  • Ubiquitin-proteasome cascade: E1 activates Ub, E2 carries Ub, E3 ligases attach Ub to substrates, DUBs remove Ub, proteasome degrades Ub-tagged substrates

Conceptual connections to prior lectures and real-world relevance

  • The signaling toolkit (receptors, kinases, phosphatases, ubiquitin system) is foundational for controlling cell fate, proliferation, and differentiation
  • Stem cell biology relies on niche signals (Wnt, BMP, Noggin, Notch, TGF-β family) to balance self-renewal and differentiation; disruptions can alter tissue homeostasis and development
  • Signaling dynamics are essential for understanding development (embryogenesis, organ formation) and regeneration, as well as pathology where these signals go awry (cancer, fibrosis, degenerative diseases)

Concept checks and thought prompts (to guide exam prep)

  • Explain how a constitutively active TK3 could alter stem cell signaling dynamics in a self-renewal pathway
  • Describe a scenario in which a phosphatase overrides a kinase’s activity and the downstream consequences for transcription factor activation
  • Compare and contrast functional vs temporary signaling pathways and provide examples of each from the material
  • Outline how ubiquitin-dependent proteolysis can regulate signaling by removing specific signaling proteins while leaving others untouched
  • Explain how receptor endocytosis contributes to termination and resensitization of signaling pathways

Citations and sources from the lecture deck

  • Core signaling principles: signal transduction pathways with receptor activation, intracellular cascades, transcriptional responses, and termination
  • Specific pathway diagrams and components: Wnt, BMP, Notch, TGF-β, Ephrin, PI3K/Akt, MAPK/ERK cascades, and transcription factor targets (e.g., c-Myc, Lef/Tcf, CREB, ERK substrates)
  • Stem cell niches: intestinal crypt signaling (Noggin, BMP, Wnt, Pten, Smads), hair follicle niche (Noggin, BMP4, Wnt), liver stem cell lineage tracing
  • Protein turnover and degradation machinery: ubiquitin ligases (E1/E2/E3), DUBs, proteasome structure (RP, CP), and substrate fate
  • Practical implications for research and medicine: how signaling dynamics inform therapeutic strategies and understanding of development and disease